Electrolyte for electrolytic capacitor and electrolytic capacitor using the same

ABSTRACT

An electrolyte for an electrolytic capacitor which is high in electrolytic conductivity, excellent in heat stability and high in withstand voltage. An electrolyte for an electrolytic capacitor including a tetrafluoroaluminate ion; and an electrolyte for an electrolytic capacitor containing a salt and a solvent, characterized in that electrolytic conductivity X (mS·cm −1 ) at 25° C. and withstand voltage Y (V) of a capacitor satisfy the relationships of formulae (I):Y≧−7.5X+150, and X≦4, Y&gt;0.

This application is a continuation of PCT/JP02/04571, filed May 10,2002.

FIELD OF THE INVENTION

The present invention relates to an electrolyte for an electrolyticcapacitor and an electrolytic capacitor using the same.

Background Art

Electrolytic capacitors have a feature such that they have a largecapacitance even in a small size, and they are widely used in a lowfrequency filter and a by-pass. The electrolytic capacitors generallyhave a structure such that an anode foil and a cathode foil are togetherspirally wound via a separator, and placed and sealed in a casing (seeFIGS. 1 and 2). As the anode foil, a metal such as aluminum or tantalum,having an insulating oxide film formed thereon as a dielectric layer isgenerally used, and as the cathode foil, an etched aluminum foil isgenerally used. For preventing an occurrence of short-circuiting betweenthe anode and the cathode, the separator disposed between the anode andthe cathode is impregnated with an electrolyte, and it functions as asubstantial cathode. Thus, the electrolyte is an important constituentwhich largely affects the properties of the electrolytic capacitor.

Among the properties of the electrolyte, an electrolytic conductivitydirectly affects the energy loss and impedance properties of theelectrolytic capacitor, and therefore, vigorous studies are being madeon the development of an electrolyte having a high electrolyticconductivity. For example, electrolytes comprising a quaternary ammoniumsalt (Japanese Prov. Patent Publication Nos. 145715/1987 and145713/1987) or a quaternary amidinium salt (International PatentPublication No. WO95/15572 and Japanese Prov. Patent Publication No.283379/1997) of phthalic acid or maleic acid dissolved in an aproticsolvent such as γ-butyrolactone, have been proposed. However, theseelectrolytes have unsatisfactory ionic mobility and unsatisfactoryanodization of the anode aluminum, and therefore, they can be used onlyin capacitors at a rated voltage of 35 V or lower in general.Specifically, in these electrolytes, generally, only those having anelectrolytic conductivity X as low as about 13 mS·cm⁻¹ or less and awithstand voltage Y as low as about 100 V or less are obtained, and theelectrolytes having an electrolytic conductivity X as relatively high as13 mS·cm⁻¹ have a withstand voltage as low as about 60 V, while theelectrolytes having a withstand voltage Y as relatively high as 100 Vhave an electrolytic conductivity as low as about 8 mS·cm⁻¹.

The electrolyte for an electrolytic capacitor is required to have higherelectrolytic conductivity, more excellent thermal stability and moreexcellent voltage proof property, and it is needed to own all of theseproperties simultaneously. Further, the electrolytic capacitor isrequired to have lower impedance, more excellent thermal stability andmore excellent voltage proof property, and it is needed to own all ofthese properties simultaneously. However, an electrolyte for anelectrolytic capacitor and an electrolytic capacitor which meet suchrequirements have not yet been realized.

DISCLOSURE OF THE INVENTION

The present inventors have conducted intensive studies to solve theabove-mentioned problems, and they have found that an excellentelectrolyte for an electrolytic capacitor and an electrolytic capacitordramatically improved in performance as compared to conventional onescan be obtained, when a selection of a salt (a cation component and ananion component) or a selection of a solvent constituting theelectrolyte is done carefully, or the electrolyte meets a specificrequirement, and the present invention has been completed.

The present invention (1) is an electrolyte for an electrolyticcapacitor comprising a tetrafluoroaluminate ion, and the electrolyte foran electrolytic capacitor, whererin the tetrafluoroaluminate ion iscontained in the form of at least one salt selected from the groupconsisting of quaternary onium salts, amine salts, ammonium salts andalkali metal salts of tetrafluoroaluminate.

Further, the present invention is an electrolytic capacitor using theelectrolyte for an electrolytic capacitor of the present invention (1),and an electrochemical element using a conductive material comprising atetrafluoroaluminate ion.

Still further, the present invention (2) is an electrolyte for anelectrolytic capacitor containing a salt and a solvent, wherein theelectrolyte satisfies the relationships of formulae (I):Y≧−7.5X+150, and X≧4, Y>0  (I)wherein X represents an electrolytic conductivity (mS·cm⁻¹) at 25° C.,and Y represents a withstand voltage (V) of a capacitor, and theelectrolyte for an electrolytic capacitor which further satisfies therelationships of formulae (II):Y≧−7.5X+150, and X≧8, Y>0  (II).

Further, the present invention is the electrolyte for an electrolyticcapacitor of the present invention (2), wherein the electrolyte contains50% by weight or more of the solvent, and a weight ratio of a solventhaving a boiling point of 250° C. or higher, a melting point of −60 to40° C. and a permittivity (ε, 25° C.) of 25 or more is equal to orlarger than a weight ratio of a solvent having a boiling point of 190°C. or higher to lower than 250° C., a melting point of −60 to 40° C. anda permittivity (ε, 25° C.) of 25 or more, in the solvent, and theelectrolyte for an electrolytic capacitor of the present invention (2),wherein the electrolyte contains 50% by weight or more of the solventand a weight ratio of a solvent having a boiling point of 190° C. orhigher to lower than 250° C., a melting point of −60 to 40° C. and apermittivity (ε, 25° C.) of 25 or more is larger than a weight ratio ofa solvent having a boiling point of 250° C. or higher, a melting pointof −60 to 40° C. and a permittivity (ε, 25° C.) of 25 or more, in thesolvent.

Still Further, the present invention is an electrolytic capacitor usingthe electrolyte for an electrolytic capacitor of the present invention(2).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an element of an electrolytic capacitorin a spirally-wound form, wherein reference numeral 1 designates ananode foil, reference numeral 2 designates a cathode foil, referencenumeral 3 designates a separator and reference numeral 4 designates alead wire.

FIG. 2 is a cross-sectional view of an electrolytic capacitor, whereinreference numeral 5 designates a sealing material and reference numeral6 designates an outer casing.

FIG. 3 is a graph showing the relationship between an electrolyticconductivity X of an electrolyte for an electrolytic capacitor of thepresent invention and a withstand voltage of an electrolytic capacitor.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, the electrolyte for an electrolytic capacitor and theelectrolytic capacitor of the present invention will be described indetail.

The first embodiment of the present invention is an electrolyte for anelectrolytic capacitor comprising a tetrafluoroaluminate ion. It hasbeen found that when a tetrafluoroaluminate ion is contained as an anioncomponent in an electrolyte for an electrolytic capacitor, theelectrolyte having high electrolytic conductivity and excellent thermalstability as well as excellent voltage proof property can be obtained.

The tetrafluoroaluminate ion is a monovalent anion comprised of fourfluorine atoms bonded to an aluminum atom, represented by the chemicalformula: AlF₄ ⁻, and it is referred to also as tetrafluoroaluminate.

The electrolyte of the present invention uses a tetrafluoroaluminate ionas all of or part of the anion component, and the amount of thetetrafluoroaluminate ion in the anion component is preferably 5 to 100mol %, more preferably 30 to 100 mol %, especially preferably 50 to 100mol %, most preferably 100 mol %.

The electrolyte of the present invention may contain atetrafluoroaluminate ion in the form of a salt in the electrolyte. It ispreferred that the tetrafluoroaluminate salt is at least one memberselected from the group consisting of quaternary onium salts, aminesalts, an ammonium salts and alkali metal salts.

Preferred examples of quaternary onium salts include quaternary ammoniumsalts, quaternary phosphonium salts, quaternary imidazolium salts andquaternary amidinium salts.

As preferred examples of quaternary ammonium ions of the quaternaryammonium salts, there can be mentioned the following.

(i) Tetraalkylammonium

Examples include tetramethylammonium, ethyltrimethylammonium,diethyldimethylammonium, triethylmethylammonium, tetraethylammonium,trimethyl-n-propylammonium, trimethylisopropylammonium,trimethyl-n-butylammonium, trimethylisobutylammonium,trimethyl-t-butylammonium, trimethyl-n-hexylammonium,dimethyldi-n-propylammonium, dimethyldiisopropylammonium,dimethyl-n-propylisopropylammonium, methyltri-n-propylammonium,methyltriisopropylammonium, methyldi-n-propylisopropylammonium,methyl-n-propyldiisopropylammonium, triethyl-n-propylammonium,triethylisopropylammonium, triethyl-n-butylammonium,triethylisobutylammonium, triethyl-t-butylammonium,dimethyldi-n-butylammonium, dimethyldiisobutylammonium,dimethyldi-t-butylammonium, dimethyl-n-butylethylammonium,dimethylisobutylethylammonium, dimethyl-t-butylethylammonium,dimethyl-n-butylisobutylammonium, dimethyl-n-butyl-t-butylammonium,dimethylisobutyl-t-butylammonium, diethyldi-n-propylammonium,diethyldiisopropylammonium, diethyl-n-propylisopropylammonium,ethyltri-n-propylammonium, ethyltriisopropylammonium,ethyldi-n-propylisopropylammonium, ethyl-n-propyldiisopropylammonium,diethylmethyl-n-propylammonium, ethyldimethyl-n-propylammonium,ethylmethyldi-n-propylammonium, diethylmethylisopropylammonium,ethyldimethylisopropylammonium, ethylmethyldiisopropylammonium,ethylmethyl-n-propylisopropylammonium, tetra-n-propylammonium,tetraisopropylammonium, n-propyltriisopropylammonium,di-n-propyldiisopropylammonium, tri-n-propylisopropylammonium,trimethylbutylammonium, trimethylpentylammonium, trimethylhexylammonium,trimethylheptylammonium, trimethyloctylammonium andtrimethylnonylammonium. These individually have 4 to 12 carbon atoms intotal, but in the electrolyte of the present invention, those having 13or more carbon atoms in total can also be used, and examples includetrimethyldecylammonium, trimethylundecylammonium andtrimethyldodecylammonium.

(ii) Aromatic Substituted Ammonium

Examples include those having 4 to 12 carbon atoms in total, such astrimethylphenylammonium, and those having 13 or more carbon atoms intotal such as tetraphenylammonium.

(iii) Aliphatic Cyclic Ammonium

Examples include pyrrolidinium such as N,N-dimethylpyrrolidinium,N-ethyl-N-methylpyrrolidinium, N,N-diethylpyrrolidinium andN,N-tetramethylenepyrrolidinium; piperidinium such asN,N-dimethylpiperidinium, N-ethyl-N-methylpiperidinium,N,N-diethylpiperidinium, N,N-tetramethylenepiperidinium andN,N-pentamethylenepiperidinium; and morpholinium such asN,N-dimethylmorpholinium, N-ethyl-N-methylmorpholinium andN,N-diethylmorpholinium. These individually have 4 to 12 carbon atoms intotal, but in the electrolyte of the present invention, those having 13or more carbon atoms in total can also be used.

(iv) Ions of Nitrogen-containing Heterocyclic Aromatic Compound

Examples include pyridinium such as N-methylpyridinium,N-ethylpyridinium, N-n-propylpyridinium, N-isopropylpyridinium andN-n-butylpyridinium. These individually have 4 to 12 carbon atoms intotal, but in the electrolyte of the present invention, those having 13or more carbon atoms in total can also be used.

Preferred examples of quaternary phosphonium ions of the quaternaryphosphonium salts include tetramethylphosphonium,triethylmethylphosphonium and tetraethylphosphonium. These individuallyhave 4 to 12 carbon atoms in total, but in the electrolyte of thepresent invention, those having 13 or more carbon atoms in total canalso be used.

Preferred examples of quaternary imidazolium ions of the quaternaryimidazolium salts include 1,3-dimethylimidazolium,1,2,3-trimethylimidazolium, 1-ethyl-3-methylimidazolium,1-ethyl-2,3-dimethylimidazolium, 1,3-diethylimidazolium,1,2-diethyl-3-methylimidazolium, 1,3-diethyl-2-methylimidazolium,1,2-dimethyl-3-n-propylimidazolium, 1-n-butyl-3-methylimidazolium,1-methyl-3-n-propyl-2,4-dimethylimidazolium,1,2,3,4-tetramethylimidazolium, 1,2,3,4,5-pentamethylimidazolium,2-ethyl-1,3-dimethylimidazolium, 1,3-dimethyl-2-n-propylimidazolium,1,3-dimethyl-2-n-pentylimidazolium, 1,3-dimethyl-2-n-heptylimidazolium,1,3,4-trimethylimidazolium, 2-ethyl-1,3,4-trimethylimidazolium,1,3-dimethylbenzimidazolium, 1-phenyl-3-methylimidazolium,1-benzyl-3-methylimidazolium, 1-phenyl-2,3-dimethylimidazolium,1-benzyl-2,3-dimethylimidazolium, 2-phenyl-1,3-dimethylimidazolium and2-benzyl-1,3-dimethylimidazolium. These are individually quaternaryimidazolium having 4 to 12 carbon atoms in total.

In the electrolyte of the present invention, quaternary imidazoliumhaving 13 or more carbon atoms in total can also be used, and preferredexamples include 1,3-dimetyl-2-n-undecylimidazolium and1,3-dimetyl-2-n-heptadecylimidazolium. Further, in the electrolyte ofthe present invention, quaternary imidazolium containing a hydroxylgroup or an ether group can also be used, and preferred examples include2-(2′-hydroxy)ethyl-1,3-dimethylimidazolium,1-(2′-hydroxy)ethyl-2,3-dimethylimidazolium,2-ethoxymethyl-1,3-dimethylimidazolium and1-ethoxymethyl-2,3-dimethylimidazolium.

Preferred examples of the quaternary amidinium include imidazoliniumsuch as 1,3-dimethylimidazolinium, 1,2,3-trimethylimidazolinium,1-ethyl-3-methylimidazolinium, 1-ethyl-2,3-dimethylimidazolinium,1,3-diethylimidazolinium, 1,2-diethyl-3-methylimidazolinium,1,3-diethyl-2-methylimidazolinium, 1,2-dimethyl-3-n-propylimidazolinium,1-n-butyl-3-methylimidazolinium,1,2,4-trimethyl-3-n-propylimidazolinium,1,2,3,4-tetramethylimidazolinium, 2-ethyl-1,3-dimethylimidazolinium,1,3-dimethyl-2-n-propylimidazolinium,1,3-dimethyl-2-n-pentylimidazolinium,1,3-dimethyl-2-n-heptylimidazolinium, 1,3,4-trimethylimidazolinium,2-ethyl-1,3,4-trimethylimidazolinium, 1-phenyl-3-methylimidazolinium,1-benzyl-3-methylimidazolinium, 1-phenyl-2,3-dimethylimidazolinium,1-benzyl-2,3-dimethylimidazolinium, 2-phenyl-1,3-dimethylimidazoliniumand 2-benzyl-1,3-dimethylimidazolinium; tetrahydropyrimidinium such as1,3-dimethyltetrahydropyrimidinium, 1,3-diethyltetrahydropyrimidinium,1-ethyl-3-methyltetrahydropyrimidinium,1,2,3-trimethyltetrahydropyrimidinium,1,2,3-triethyltetrahydropyrimidinium,1-ethyl-2,3-dimethyltetrahydropyrimidinium,2-ethyl-1,3-dimethyltetrahydropyrimidinium,1,2-diethyl-3-methyltetrahydropyrimidinium,1,3-diethyl-2-methyltetrahydropyrimidinium,5-methyl-1,5-diazabicyclo[4.3.0]nonenium-5 and8-methyl-1,8-diazabicyclo[5.4.0]undecenium-7. These are individuallyquaternary amidinium having 4 to 12 carbon atoms in total.

In the electrolyte of the present invention, quaternary amidinium having13 carbon atoms or more in total can also be used, and preferredexamples include 1,3-dimethyl-2-n-undecylimidazolinium and1,3-dimethyl-2-n-heptadecylimidazolinium. Further, in the electrolyte ofthe present invention, quaternary amidinium containing a hydroxyl groupor an ether group can also be used, and preferred examples include2-(2′-hydroxy)ethyl-1,3-dimethylimidazolinium,1-(2′-hydroxy)ethyl-2,3-dimethylimidazolinium,2-ethoxymethyl-1,3-dimethylimidazolinium and1-ethoxymethyl-2,3-dimethylimidazolinium.

The electrolyte of the present invention may contain atetrafluoroaluminate ion in the form of not only a quaternary onium saltbut also an amine salt, an ammonium salt (NH₄ ⁺AlF₄ ⁻) or an alkalimetal salt.

Preferred examples of amines of the amine salt include tertiary aminessuch as trimethylamine, ethyldimethylamine, diethylmethylamine,triethylamine, pyridine, N-methylimidazole,1,5-diazabicyclo[4.3.0]nonene-5 and 1,8-diazabicyclo[5.4.0]undecene-7.In addition to the tertiary amines, primary amines and secondary aminescan be used, and examples include diethylamine, diisopropylamine,isobutylamine, di-2-ethylhexylamine, pyrrolidine, piperidine,morpholine, hexamethyleneimine, ethylamine, n-propylamine,isopropylamine, t-butylamine, sec-butylamine, 2-ethylhexylamine,3-methoxypropylamine and 3-ethoxypropylamine. Preferred examples ofalkali metals include lithium, sodium, potassium, rubidium and cesium.

Among these cation components, from the viewpoint of obtaining anelectrolyte having high electrolytic conductivity, quaternary oniumhaving 4 to 12 carbon atoms in total are preferred, and of these,preferred is at least one compound selected from the group consisting oftetraethylammonium, triethylmethylammonium, diethyldimethylammonium,ethyltrimethylammonium, tetramethylammonium, N,N-dimethylpyrrolidinium,N-ethyl-N-methylpyrrolidinium, 1,3-dimethylimidazolium,1,2,3-trimethylimidazolium, 1-ethyl-3-methylimidazolium,1-ethyl-2,3-dimethylimidazolium, 1,2,3,4-tetramethylimidazolium,1,3-diethylimidazolium, 2-ethyl-1,3-dimethylimidazolium,1,3-dimethyl-2-n-propylimidazolium, 1,3-dimethyl-2-n-pentylimidazolium,1,3-dimethyl-2-n-heptylimidazolium, 1,3,4-trimethylimidazolium,2-ethyl-1,3,4-trimethylimidazolium, 1,3-dimethylbenzimidazolium,1-phenyl-3-methylimidazolium, 1-benzyl-3-methylimidazolium,1-phenyl-2,3-dimethylimidazolium, 1-benzyl-2,3-dimethylimidazolium,2-phenyl-1,3-dimethylimidazolium, 2-benzyl-1,3-dimethylimidazolium,1,3-dimethylimidazolinium, 1,2,3-trimethylimidazolinium,1-ethyl-3-methylimidazolinium, 1-ethyl-2,3-dimethylimidazolinium,1,2,3,4-tetramethylimidazolinium, 1,3-diethylimidazolinium,2-ethyl-1,3-dimethylimidazolinium, 1,3-dimethyl-2-n-propylimidazolinium,1,3-dimethyl-2-n-pentylimidazolinium,1,3-dimethyl-2-n-heptylimidazolinium, 1,3,4-trimethylimidazolinium,2-ethyl-1,3,4-trimethylimidazolinium, 1-phenyl-3-methylimidazolinium,1-benzyl-3-methylimidazolinium, 1-phenyl-2,3-dimethylimidazolinium,1-benzyl-2,3-dimethylimidazolinium, 2-phenyl-1,3-dimethylimidazoliniumand 2-benzyl-1,3-dimethylimidazolinium, and more preferred is1-ethyl-2,3-dimethylimidazolinium or 1,2,3,4-tetramethylimidazolinium.

The electrolyte of the present invention may contain an anion componentother than the tetrafluoroaluminate ion, and specific examples include afluorine-containing inorganic ion such as a tetrafluoroborate ion, ahexafluorophosphate ion, a hexafluoroarsenate ion, ahexafluoroantimonate ion, a hexafluoroniobate ion and ahexafluorotantalate ion; a carboxylate ion such as a phthalate ion, amaleate ion, a salicylate ion, a benzoate ion and an adipate ion; asulfonate ion such as a benzenesulfonate ion, a toluenesulfonate ion, adodecylbenzenesulfonate ion, a trifluoromethanesulfonate ion and aperfluorobutanesulfonate ion; an inorganic oxoacid ion such as a borateion and a phosphate ion; a bis(trifluoromethanesulfonyl)imide ion; abis(pentafluoroethanesulfonyl)imide ion; atris(trifluoromethanesulfonyl)methide ion; a perfluoroalkylborate ion;and a perfluoroalkylphosphate ion. As the salt, a hydrogenphthalate, ahydrogenmaleate, etc. can be used. For example, when ahydrogenphthalate, a hydrogenmaleate, etc. is used in combination with atetrafluoroaluminate, it is preferred that the tetrafluoroaluminate is amain component, and the amount of the tetrafluoroaluminate is preferably50% by weight or more, more preferably 60% by weight or more, furtherpreferably 70% by weight or more based on the total weight of the salts,and a larger tetrafluoroaluminate amount is preferred.

When the tetrafluoroaluminate in the present invention is used in anelectrolytic capacitor, it is required that the tetrafluoroaluminatehave a high purity, and therefore, the tetrafluoroaluminate is usedafter purified by recrystallization or extraction with a solvent so thatit has a desired purity, if necessary.

The concentration of the tetrafluoroaluminate in the electrolyte of thepresent invention is preferably 5 to 40% by weight, further preferably10 to 35% by weight. The reason for this is as follows: when thetetrafluoroaluminate concentration is too low, only a low electrolyticconductivity can be obtained, and when the tetrafluoroaluminateconcentration is too high, an increase of the electrolyte in viscosityor deposition of a salt at low temperatures is likely to occur, etc.Generally, the lower the concentration is, the withstand voltage of anelectrolyte for electrolytic capacitor tend to become higher, and hencethe optimum concentration can be determined from a desired rated voltageof the capacitor. The electrolyte of the present invention may be eithera high-concentration solution containing 50% or more of a salt or anormal temperature molten salt.

From the viewpoint of obtaining an electrolyte having further excellentelectrolytic conductivity and thermal stability as well as voltage proofproperty, it is preferred that the electrolyte of the present inventioncontains 50% by weight or more of a solvent. As the solvent, there canbe mentioned at least one solvent selected from the group consisting ofcarbonic esters, carboxylic esters, phosphoric esters, nitrites, amides,sulfones, alcohols and water, and it is preferred that the solvent isselected from carbonic esters, carboxylic esters, phosphoric esters,nitrites, amides, sulfones and alcohols, which tend to exhibit stableproperties with a lapse of time when used in an electrolyte. When wateris used as a solvent, it is preferred that water and another solvent areused in combination, namely, water is used as part of the solvent.

Specific examples of solvents include the following: carbonic esterssuch as linear carbonic esters (e.g., linear carbonic esters such asdimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, diphenylcarbonate and methylphenyl carbonate) and cyclic carbonic esters (e.g.,cyclic carbonic esters such as ethylene carbonate, propylene carbonate,ethylene 2,3-dimethylcarbonate, butylene carbonate, vinylene carbonateand ethylene 2-vinylcarbonate); carboxylic esters such as aliphtaticcarboxylic esters (e.g., methyl formate, methyl acetate, methylpropionate, ethyl acetate, propyl acetate, butyl acetate and amylacetate), aromatic carboxylic esters (e.g., aromatic carboxylic esterssuch as methyl benzoate and ethyl benzoate) and lactones (e.g.,γ-butyrolactone, γ-valerolactone and δ-valerolactone); phosphoric esterssuch as trimethyl phosphate, ethyldimethyl phosphate, diethylmethylphosphate and triethyl phosphate; nitriles such as acetonitrile,propionitrile, methoxypropionitrile, glutaronitrile, adiponitrile and2-methylglutaronitrile; amides such as N-methylformamide,N-ethylformamide, N,N-dimethylformamide, N,N-dimethylacetamide andN-methylpyrrolidinone; sulfones such as dimethyl sulfone, ethylmethylsulfone, diethyl sulfone, sulfolane, 3-methylsulfolane and2,4-dimethylsulfolane; alcohols such as ethylene glycol, propyleneglycol, ethylene glycol monomethyl ether and ethylene glycol monoethylether; ethers such as ethylene glycol dimethyl ether, ethylene glycoldiethyl ether, 1,4-dioxane, 1,3-dioxolane, tetrahydrofuran,2-methyltetrahydrofuran, 2,6-dimethyltetrahydrofuran andtetrahydropyran; sulfoxides such as dimethyl sulfoxide, methylethylsulfoxide and diethyl sulfoxide; 1,3-dimethyl-2-imidazolidinone;1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone; and3-methyl-2-oxazolidinone.

From the viewpoint of obtaining an electrolyte having more excellentelectrolytic conductivity, a non-aqueous solvent having a permittivity(ε, 25° C.) of 25 or more can be preferably used, and from the viewpointof safety, a non-aqueous solvent having a flash point of 70° C. orhigher can be preferably used.

From the viewpoint of obtaining an electrolyte having more excellentthermal stability, the solvent preferably contains a solvent having aboiling point of 250° C. or higher, a melting point of −60 to 40° C. anda permittivity (ε, 25° C.) of 25 or more in an amount of 25% by weightor more, more preferably 40% by weight or more, especially preferably50% by weight or more, based on the total weight of the solvents.Examples of such solvents include sulfones, and especially preferred aresulfolane and 3-methylsulfolane. By using these solvents in combinationin the electrolyte, there can be obtained an electrolytic capacitorhaving low impedance and high withstand voltage which ensures that itcan operate for 1,000 hours or longer at an environment temperature of110 to 150° C.

Further, from the viewpoint of obtaining an electrolytic capacitorhaving lower impedance, the solvent preferably contains a solvent havinga boiling point of 190° C. or higher to lower than 250° C., a meltingpoint of −60 to 40° C. and a permittivity (ε, 25° C.) of 25 or more inan amount of 25% by weight or more, more preferably 40% by weight ormore, especially preferably 50% by weight or more, based on the totalweight of the solvents. Examples of such solvents include carbonicesters, carboxylic esters, phosphoric esters, nitriles, amides andalcohols, and especially preferred are γ-butyrolactone and ethyleneglycol. By using these solvents in combination in the electrolyte, anelectrolytic capacitor having extremely low impedance and high withstandvoltage can be obtained.

As especially preferred electrolytes, from the viewpoint of achievingexcellent thermal stability, there can be mentioned electrolytes for anelectrolytic capacitor, wherein sulfolane is a solvent and1-ethyl-2,3-dimethylimidazolinium tetrafluoroaluminate or1,2,3,4-tetramethylimidazolinium tetrafluoroaluminate in an amount of 5to 40% by weight based on the total weight of the electrolyte is addedthereto; from the viewpoint of obtaining an electrolytic capacitorhaving low impedance, there can be mentioned electrolytes for anelectrolytic capacitor, wherein γ-butyrolactone is a solvent and1-ethyl-2,3-dimethylimidazolinium tetrafluoroaluminate or1,2,3,4-tetramethylimidazolinium tetrafluoroaluminate in an amount of 5to 40% by weight based on the total weight of the electrolyte is addedthereto. A solvent using both sulfolane and γ-butyrolactone is alsopreferred.

In the electrolyte of the present invention, in addition to the salt andthe solvent, different additives may be used. There are various objectsfor adding an additive to the electrolyte, and there can be mentionedthe improvement of electrolytic conductivity, the improvement of thermalstability, suppression of deterioration of an electrode due to hydrationor dissolution, suppression of gas generation, the improvement ofvoltage proof and the improvement of wettability. With respect to thecontent of an additive, there is no particular limitation, but theadditive content is preferably in the range of from 0.1 to 20% byweight, more preferably in the range of from 0.5 to 10% by weight.

Examples of such additives include nitro compounds such asp-nitrophenol, m-nitroacetophenone and p-nitrobenzoic acid; phosphoruscompounds such as dibutyl phosphate, monobutyl phosphate, dioctylphosphate, monooctyl octylphosphonate and phosphoric acid; boroncompounds such as complex compounds of boric acid and a polyhydricalcohol (e.g., ethylene glycol, glycerol, mannitol and polyvinylalcohol); metal oxide fine particles such as silica and aluminosilicate;polyalkylene glycols such as polyethylene glycol and polypropyleneglycol, and copolymers thereof; and surfactants such as silicone oil.

The electrolyte of the present invention may be used in the form of whatis called gelled electrolyte obtained by solidification by adding apolymer compound to the electrolyte. Examples of polymers used in thegelled electrolyte include polyethylene oxides, polyacrylonitriles,polytetrafluoroethylenes, polyvinylidene fluorides and polymethylmethacrylates.

In the electrolyte of the present invention, when a non-aqueous solventis used as a solvent for the electrolyte, the moisture content iscontrolled so that the life properties of a capacitor using such anelectrolyte are more stable. It is generally known that, when a largeamount of moisture is contained in the electrolyte of an electrolyticcapacitor using a non-aqueous solvent, aluminum used in an anode or acathode deteriorates due to hydration and gas generates at the sametime, during the use of the capacitor for a long term. It is also knownthat, when the electrolyte contains no moisture, the anodization inrestoration of the anode oxide film is likely to be lowered.

However, conventional electrolytes and capacitors have been used in aregion of the rated voltage as low as 35 V or lower, and therefore theeffect on the life properties of the capacitor is small even when about3% by weight of moisture is present. On the other hand, the capacitorusing the electrolyte of the present invention can be used in a regionof the rated voltage as high as 100 V and meets a requirement of highheat resistance, and therefore the effect of the moisture content islarge as compared to the conventional ones. When a non-aqueous solventis used, the electrolyte of the present invention preferably has amoisture content in the electrolyte of 1% by weight or less, andpreferably 0.01 to 1% by weight, especially preferably 0.01 to 0.1% byweight, taking the above anodization into consideration.

The present invention also provides an electrolytic capacitor using theelectrolyte of the present invention. Examples of electrolyticcapacitors include aluminum electrolytic capacitors, tantalumelectrolytic capacitors and niobium electrolytic capacitors. Withrespect to the structure and material of the electrolytic capacitor,there is no particular limitation as long as the electrolyte of thepresent invention is used. Therefore, all the conventionally usedelectrolytic capacitors and recently proposed electrolytic capacitorsusing the electrolyte of the present invention are involved within thescope of the present invention.

In the aluminum electrolytic capacitor of the present invention, forexample, an element formed by spirally winding an anode foil and acathode foil via a separator paper is used. As the anode foil, analuminum foil having a purity of 99.9% which is subjected to surfacetreatment by chemical or electrochemical etching in an acid solution andis then subjected to anodization treatment in an aqueous solution ofammonium adipate, boric acid or phosphoric acid to form a layer of analuminum oxide film on the surface, may be used. As the cathode foil, analuminum foil having a purity of 99.9% which is subjected to surfacetreatment may be used. Further, as the cathode foil, an etched aluminumfoil having a titanium nitride thin film formed on its surface (forexample, described in Japanese Prov. Patent Publication No. 186054/1997)may be used. The separator in the capacitor element having such aconstruction is impregnated with the electrolyte of the presentinvention. The element having the separator impregnated with theelectrolyte is stored in a cylindrical outer casing with a bottomcomprised of aluminum, and a sealing material made of a butyl rubber isinserted into the opening end portion of the outer casing, and then theelectrolytic capacitor is sealed by drawing-processing the end portionof the outer casing to obtain an aluminum electrolytic capacitor. It ismore preferred that the surface of the sealing material is coated with aresin such as Teflon, or is covered with a sheet such as bakelite, sincethe transmission of solvent vapor is lowered.

As the separator, paper such as manila paper and kraft paper isgenerally used, but nonwoven fabric of glass fiber, polypropylene orpolyethylene can also be used. As the butyl rubber used in the sealingmaterial, there can be used an elastomer which is obtained by adding areinforcing agent (e.g., carbon black), a bulking agent (e.g., clay,talc and calcium carbonate), a processing aid (e.g., stearic acid andzinc oxide), a vulcanizing agent, etc. to a raw rubber comprised of acopolymer of isobutylene and isoprene and kneading them, and thenrolling and molding the resultants. As the vulcanizing agent,alkylphenol-formalin resins; peroxides (e.g., dicumyl peroxide,1,1-di-(t-butylperoxy)-3,3,5-trimethylcyclohexane and2,5-dimethyl-2,5-di-(t-butylperoxy)hexane); quinoids (e.g.,p-quinonedioxime and p,p′-dibenzoylquinonedioxime); and sulfur can beused.

The aluminum electrolytic capacitor of the present invention may have ahermetic sealing structure or a structure such that a resin casingcontaining the capacitor is sealed (for example, described in JapaneseProv. Patent Publication No. 148384/1996). In case of the aluminumelectrolytic capacitor having a rubber sealing structure, gas permeatesthrough the rubber to some extent, and therefore a solvent volatilizesfrom the inside of the capacitor to the air in a high-temperatureenvironment, or moisture enters the capacitor from the air in ahigh-temperature high-humidity environment. In such severe environments,the capacitor suffers unfavorable changes in properties, e.g., loweringof the capacitance. On the other hand, in case of the capacitor having ahermetic sealing structure or a structure such that a resin casingcontaining the capacitor is sealed, the gas transmission is extremelysmall, and therefore the capacitor exhibits stable properties even inthe severe environments.

The second embodiment of the present invention is an electrolyte for anelectrolytic capacitor containing a salt and a solvent, wherein theelectrolyte satisfies the relationships of formulae (I):Y≧−7.5X+150, and X≧4, Y>0  (I)wherein X represents an electrolytic conductivity (mS·cm⁻¹) at 25° C.,and Y represents a withstand voltage (V) of a capacitor.

When the types and concentrations of the salt and the solvent which areconstituents of the electrolyte are selected so that the electrolyticconductivity X and the withstand voltage Y satisfy the relationships offormulae (I), an electrolytic capacitor having low impedance andexcellent voltage proof property can be obtained.

The electrolytic conductivity X in formulae (I) is an electrolyticconductivity X (mS·cm⁻¹) of an electrolyte at 25° C., and can bemeasured using an electrolytic conductivity meter. In the presentinvention, the electrolytic conductivity X is preferably 4 mS·cm⁻¹ ormore, more preferably 8 mS·cm⁻¹ or more. Because the use of anelectrolyte having a higher electrolytic conductivity makes it possibleto obtain an electrolytic capacitor having lower impedance or lowerequivalent series resistance. The upper limit of the electrolyticconductivity is desirably higher, but generally about 30 mS·cm⁻¹.

The withstand voltage Y in formulae (I) is a withstand voltage of anelectrolytic capacitor, and defined as a voltage value at which thefirst spike or scintillation is observed in a voltage-time ascendingcurve obtained when applying a constant current to the electrolyticcapacitor. In the present invention, a method for measuring thewithstand voltage Y is as follows.

Measuring method: As an aluminum electrolytic capacitor element, acapacitor having a spirally-wound form structure (casing size: 10φ×20 L;rated voltage: 200 V; capacitance: 20 μF) is used (FIG. 1). An aluminumelectrolytic capacitor having a structure such that the spirally-woundform element is impregnated with an electrolyte and then placed in analuminum outer casing, and the casing is sealed by a butyl rubbervulcanized by a peroxide, is prepared (FIG. 2). A constant current of 10mA is applied to the capacitor at 125° C. to obtain a voltage-timecurve, thus determining a withstand voltage Y.

In the present invention, the withstand voltage Y may be more than 0 V,preferably 50 V or more, further preferably 100 V or more. Because ahigher withstand voltage makes it possible to prepare an electrolyticcapacitor having a higher rated voltage, and the safety is improved whentoo high a voltage is applied to the electrolytic capacitor by misuse.The upper limit of the withstand voltage is desirably higher, butgenerally about 300 V.

With respect to the combination of the salt and the solvent contained inthe electrolyte of the present invention, there is no particularlimitation as long as the formulae (I) above are satisfied. Examplesinclude those having a tetrafluoroaluminate (e.g., a quaternary oniumsalt, an amine salt, an ammonium salt and an alkali metal salt oftetrafluoroaluminate) or the combination of the tetrafluoroaluminate anda hydrogenphthalate, a hydrogenmaleate, etc. as a salt, and at least onemember selected from the group consisting of carbonic esters, carboxylicesters, phosphoric esters, nitrites, amides, sulfones, alcohols andwater as a solvent.

The tetrafluoroaluminate is a salt comprising a tetrafluoroaluminate ionas an anion component as mentioned in connection with the firstembodiment. Specific examples include quaternary onium salts, aminesalts, ammonium salts and alkali metal salts of tetrafluoroaluminate,and specific examples and preferred examples of cation components ofthese salts include those mentioned in connection with the firstembodiment. When a tetrafluoroaluminate is used, the electrolyte maycontain an anion component other than the tetrafluoroaluminate ion, andspecific examples of other anion components include those mentioned inconnection with the first embodiment. The amount of thetetrafluoroaluminate ion in the anion component is preferably 5 to 100mol %, more preferably 30 to 100 molt, especially preferably 50 to 100molt, most preferably 100 mol %. When a tetrafluoroaluminate and ahydrogenphthalate, a hydrogenmaleate, etc. are used in combination as asalt, it is preferred that the tetrafluoroaluminate is a main component,and the amount of the tetrafluoroaluminate is preferably 50% by weightor more, more preferably 60% by weight or more, further preferably 70%by weight or more based on the total weight of the salts, and a largertetrafluoroaluminate amount is preferred.

The concentration of the salt used in the electrolyte of the presentinvention is preferably 5 to 40% by weight, more preferably 10 to 35% byweight, but, generally, the lower the salt concentration is, thewithstand voltage of an electrolyte tend to become higher so that theoptimum concentration may be determined from a desired rated voltage ofthe capacitor. When the salt is used in an electrolytic capacitor, it isnecessary that the salt have a high purity, and therefore the salt isused after purified by recrystallization or extraction with a solvent sothat it has a desired purity, if necessary.

As the solvent, as mentioned above, there can be mentioned at least onemember selected from the group consisting of carbonic esters, carboxylicesters, phosphoric esters, nitrites, amides, sulfones, alcohols andwater, and specific examples and preferred examples of solvents includethose mentioned in connection with the first embodiment. It is preferredthat the amount of the solvent in the electrolyte is 50% by weight ormore, and from the viewpoint of safety, it is preferred that the solventcontains a non-aqueous solvent having a flash point of 70° C. or higher.

From the viewpoint of properties of an electrolytic capacitor, as morepreferred embodiments, the electrolyte of the second embodiment of thepresent invention is classified into two embodiments, i.e., theelectrolyte, wherein the main component of the solvent belongs to a highboiling-point solvent group, and the electrolyte, wherein the maincomponent of the solvent belongs to a low boiling-point solvent group.

The case, wherein the main component of the solvent belongs to a highboiling-point solvent means, when the solvent contained in theelectrolyte is classified into a high boiling-point solvent group{boiling point: 250° C. or higher; melting point: −60 to 40° C.;permittivity (ε, 25° C.): 25 or more}, a low boiling-point solvent group{boiling point: 190° C. or higher to lower than 250° C.; melting point:−60 to 40° C.; permittivity (ε, 25° C.): 25 or more} and a solvent groupwhich does not belong to any of the above groups for convenience, thecase, wherein the weight ratio of the solvent belonging to the highboiling-point solvent group is equal to or larger than the weight ratioof the solvent belonging to the low boiling-point solvent group. Thisinvolves a mixed solvent in which all the solvents belong to the highboiling-point solvent group and no solvent belonging to the lowboiling-point solvent group is contained, and a single solvent belongingto the high boiling-point solvent group. On the other hand, the case,wherein the main component of the solvent belongs to a low boiling-pointsolvent group means the case, wherein the weight ratio of the solventbelonging to the low boiling-point solvent group is larger than theweight ratio of the solvent belonging to the high boiling-point solventgroup. This involves a mixed solvent in which all the solvents belong tothe low boiling-point solvent group and no solvent belonging to the highboiling-point solvent group is contained, and a single solvent belongingto the low boiling-point solvent group. In the electrolyte of thepresent invention, the solvent mainly belongs to either the highboiling-point solvent group or the low boiling-point solvent group, anda solvent which does not belong to any of these groups is present as aminor component, generally, in an amount of 40% by weight or less.

(1) The case, wherein the main component of the solvent belongs to thehigh boiling-point solvent

In the electrolyte which satisfies the formulae (I), when theelectrolyte contains 50% by weight or more of a solvent and the maincomponent of the solvent belongs to the high boiling-point solvent group(boiling point: 250° C. or higher; melting point: −60 to 40° C.;permittivity (ε, 25° C.): 25 or more), an electrolytic capacitor havingespecially excellent thermal stability can be obtained using thiselectrolyte. From the viewpoint of achieving excellent thermalstability, the amount of the solvent belonging to the high boiling-pointsolvent group is preferably 60% by weight or more, more preferably 70%by weight or more, especially preferably 100% by weight based on thetotal weight of the solvents. Examples of solvents used in theelectrolyte include sulfones, and especially preferred are sulfolane and3-methylsulfolane. By using such an electrolyte, there can be obtainedan electrolytic capacitor having low impedance and high withstandvoltage, which ensures that it can operate for 1,000 hours or longer atan environment temperature of 110 to 150° C.

(2) The case, wherein the main component of the solvent has relativelylow boiling-point

In the electrolyte which satisfies the formulae (I), when theelectrolyte contains 50% by weight or more of a solvent and the maincomponent of the solvent belongs to the low boiling-point solvent group(boiling point: 190° C. or higher to lower than 250° C.; melting point:−60 to 40° C.; permittivity (ε, 25° C.): 25 or more), an electrolyticcapacitor having especially low impedance can be obtained using thiselectrolyte. From the viewpoint of obtaining an electrolytic capacitorhaving low impedance, the amount of the solvent belonging to the lowboiling-point solvent group is preferably 60% by weight or more, morepreferably 70% by weight or more, especially preferably 100% by weightbased on the total weight of the solvents. It is further preferred thatthe electrolyte satisfies the relationship of formula (III):Y≧−7.5X+220  (III).

Examples of solvents as a main component used in the electrolyte includeat least one solvent selected from the group consisting of carbonicesters, carboxylic esters, phosphoric esters, nitrites, amides andalcohols, and especially preferred are γ-butyrolactone and ethyleneglycol. By using such an electrolyte, there can be obtained anelectrolytic capacitor having extremely low impedance and high withstandvoltage.

In the electrolyte of the second embodiment of the present invention,from the viewpoint of properties of a capacitor (impedance, withstandvoltage, thermal stability, life, reliability, etc.), especiallypreferred combinations of the salt and the solvent are the combinationof 1-ethyl-2,3-dimethylimidazolinium tetrafluoroaluminate or1,2,3,4-tetramethylimidazolinium tetrafluoroaluminate as a salt andsulfolane as a solvent, and the combination of1-ethyl-2,3-dimethylimidazolinium tetrafluoroaluminate or1,2,3,4-tetramethylimidazolinium tetrafluoroaluminate as a salt andγ-butyrolactone as a solvent. A solvent using both sulfolane andγ-butyrolactone is also preferred.

In the electrolyte of the present invention, in addition to the salt andthe solvent, different additives may be used. Specific examples andpreferred examples of additives and amounts of the additive addedinclude those mentioned in connection with the first embodiment. In thesecond embodiment, the electrolytic conductivity and withstand voltagevalues are those measured with respect to an original electrolytecomprising a salt and a solvent, which contains no additive. When theoriginal electrolyte satisfies the formulae (I), (II) and (III), whichfurther contains an additive in case of necessity, is also involvedwithin the scope of the present invention.

In the electrolyte of the present invention, when a non-aqueous solventis used as a solvent for the electrolyte, the moisture content iscontrolled, so that the life properties of a capacitor using such anelectrolyte are more stable. The control of the moisture content is thesame as that mentioned above in connection with the first embodiment.

The present invention also provides an electrolytic capacitor using theelectrolyte of the second embodiment. The method for producing anelectrolytic capacitor and parts of the electrolytic capacitor are thesame as those mentioned above in connection with the first embodiment.

EXAMPLES

Hereinbelow, the present invention will be described in more detail withreference to the following Examples. These examples should not beconstrued as limiting the scope of the present invention. The materials,ratios, formulations and procedure in the following Examples can beappropriately changed as long as the aim of the present invention is notsacrificed.

(1) Synthesis of Triethylmethylammonium Tetrafluoroaluminate

6.90 g (50.0 mmol) of aluminum fluoride trihydrate was weighed andplaced in a round flask made of PFA, and air in the flask was evacuatedand then replaced by argon gas. While feeding argon gas to the flaskthrough its inlet, 100 ml of dehydrated acetonitrile was added and theflask was sealed followed by stirring for 30 minutes. Then, whilefeeding argon gas similarly, 9.46 g (50.0 mmol) oftriethylmethylammonium fluoride trihydrate was added portion wise, andthe resultant mixture was further stirred for about 3 hours. Theunreacted solid was filtered off, and then the solvent was distilled offto obtain about 9 g of white crude crystal of triethylmethylammoniumtetrafluoroaluminate. This was subjected to recrystallization from 10 gof isopropanol. The amount of the product was 5.50 g, and the yield was50% based on the charged raw material. Identification was made byelemental analysis and NMR, and a melting point was measured by TG-DTA.

Elemental analysis: Theoretical value: C: 38.36, H: 8.28, N: 6.39, Al:12.31, F: 34.67 Analysis value: C: 38.40, H: 7.70, N: 6.32, Al: 12.0, F:33.50 ¹⁹F-NMR: −190 ppm {sixtuplet, J=34 Hz, CFCl₃ reference in (CD₃)₂SOsolvent}²⁷Al-NMR: 49 ppm {quintuplet, J=34 Hz, AlCl₃.3H₂O reference in(CD₃)₂SO solvent}Melting point: 320° C. (decomposed)

(2) Preparation of Aluminum Electrolytic Capacitor, and Evaluation ofElectrolytic Conductivity and Voltage Proof Property

Example 1, Comparative Examples 1 and 2

In Example 1, the above-obtained triethylmethylammoniumtetrafluoroaluminate was dissolved in γ-butyrolactone to prepare anelectrolyte having a concentration of 25% by weight. With respect to theelectrolyte in Example 1, an electrolytic conductivity (25° C.) of theelectrolyte immediately after the preparation and an electrolyticconductivity of the electrolyte subjected to heating test at 125° C. for25 hours were individually measured. Then, an aluminum electrolyticcapacitor having a structure such that the spirally-wound form elementshown in FIG. 1 was impregnated with the electrolyte and placed in analuminum outer casing, and the casing was sealed by a butyl rubbervulcanized by a peroxide was prepared (FIG. 2). The withstand voltagevalue was determined as a voltage value at which the first spike orscintillation was observed in a voltage-time ascending curve obtained byapplying a constant current of 10 mA to the electrolytic capacitor at125° C. Specifications of the aluminum electrolytic capacitor used weresuch that the casing size was 10φ×20 L, the rated voltage was 200 V andthe capacitance was 20 μF. Further, in Comparative Examples,electrolytes were prepared in accordance with the same procedure as inExample 1 except that triethylmethylammoniumhydrogenphthalate-(Comparative Example 1) and1-ethyl-2,3-dimethylimidazolinium hydrogenphthalate (Comparative Example2) were used respectively as a salt, and the respective evaluations wereconducted. The results are shown in Table 1.

TABLE 1 Electrolytic conductivity/ Concen- mS cm⁻¹ WithstandExperimental tration Initial After voltage Example Salt wt % valueheating V Example 1 Et₃MeN⁺ AlF₄ ⁻ 25 21.00 21.00 165 ComparativeEt₃MeN⁺ PH⁻ 25 10.86 10.77 60 example 1 Comparative EDMI⁺ PH⁻ 25 11.8910.84 55 example 2 *EDMI⁺ denotes a 1-ethyl-2,3-dimethylimidazoliniumion, and PH⁻ denotes a hydrogenphthalate ion.

From a comparison between Example and Comparative Examples, it is foundthat the electrolytic conductivity in Example 1 is as high as about twotimes those in Comparative Examples, and the change of the electrolyticconductivity in Example 1 by heating is small, indicating that thethermal stability is excellent. In addition, the withstand voltage valuein Example 1 is as high as about 2.5 to 3 times those in ComparativeExamples.

(3) Other Examples

Ingredients were mixed together by the formulations shown in Table 2mentioned below to prepare electrolytes. In the Table, part(s) for theingredients was given by weight. With respect to the electrolytesobtained, an electrolytic conductivity and a withstand voltage propertywere evaluated. First, an electrolytic conductivity at 25° C. wasmeasured. Then, a withstand voltage value at 125° C. was measured in thesame manner as in Example 1. The results are shown in Table 2.

TABLE 2 Electrolytic conductivity/ Concen- mS cm⁻¹ WithstandExperimental tration Initial After voltage Example Salt wt % valueheating V Example 2 Et₃MeN⁺ AlF₄ ⁻ 10 11.08 11.08 195 ComparativeEt₃MeN⁺ PH⁻ 14 8.17 7.98 75 example 3 Comparative EDMI⁺ PH⁻ 10 7.13 6.6080 example 4 *PH⁻ denotes a hydrogenphthalate ions, and EDMI⁺ denotes a1-ethyl-2,3-dimethylimidazolinium ion.

From a comparison between Example and Comparative Examples in Table 2,it is found that Example has a higher electrolytic conductivity and ahigher withstand voltage. Therefore, the electrolyte of the presentinvention is preferred in aluminum electrolytic capacitors in anyapplications for low impedance grade or high rated voltage.

Examples 3 to 15 and Comparative Examples 5 to 10

Further, in Examples 3 to 14, ingredients were mixed together by theformulations shown in Table 3 to prepare electrolytes. In the Table,part(s) for the ingredients was given by weight. As silica, an ethyleneglycol sol of silica having an average particle size of about 25 nm wasused. Further, electrolytes in Comparative Examples 5 to 10 wereprepared using, instead of 1-ethyl-2,3-dimethylimidazoliniumtetrafluoroaluminate used in Examples 3 to 8,1-ethyl-2,3-dimethylimidazolinium hydrogenphthalate in the same amountin terms of parts by weight.

With respect to the electrolytes obtained in Examples 3 to 14 andComparative Examples 5 to 10, an electrolytic conductivity (25° C.)immediately after the preparation was measured. Then, using therespective electrolytes, aluminum electrolytic capacitors each having arated voltage of 200 V and a capacitance of 20° F. were prepared in thesame manner as in Example 1, and a withstand voltage value (125° C.) wasmeasured. The results are shown in Table 3.

TABLE 3 Example 3 Example 4 Example 5 Example 6 Example 7 Example 81-Ethyl-2,3- 25 25 25 25 20 20 dimethylimidazoliniumtetrafluoroaluminate 1,2,3,4- Tetramethylimidazoliniumtetrafluoroaluminate 1-Ethyl-2,3- dimethylimidazoliniumhydrogenphthalate γ-Butyrolactone 75 35.5 72 72 Sulfolane 75 35.5 35.53-Methylsulfolane 35.5 Ethylene glycol 8 8 Silica 6 Phosphoric acidBoric acid p-Nitro benzoic acid Polyethylene glycol (Average molecularweight: 300) Electrolytic conductivity/ 24.10 6.56 14.41 5.94 20.5019.38 mS cm⁻¹ at 25° C. Withstand voltage/V at 160 160 165 170 170 185125 ° C. Comp. Comp. Comp. Comp. Comp. Comp. ex. 5 ex. 6 ex. 7 ex. 8 ex.9 ex. 10 1-Ethyl-2,3- dimethylimidazolinium tetrafluoroaluminate1,2,3,4- Tetramethylimidazolinium tetrafluoroaluminate 1-Ethyl-2,3- 2525 25 25 20 20 dimethylimidazolinium hydrogenphthalate γ-Butyrolactone75 35.5 72 72 Sulfolane 75 35.5 35.5 3-Methylsulfolane 35.5 Ethyleneglycol 8 8 Silica 6 Electrolytic conductivity/ 11.70 3.24 7.34 2.94 9.869.31 mS cm⁻¹ at 25° C. Withstand voltage/V at 60 55 60 60 70 80 125° C.Example 9 Example 10 Example 11 Example 12 Example 13 Example 141-Ethyl-2,3- 25 25 25 25 12.5 dimethylimidazolinium tetrafluoroaluminate1,2,3,4- 25 Tetramethylimidazolinium tetrafluoroaluminate 1-Ethyl-2,3-12.5 dimethylimidazolinium hydrogenphthalate γ-Butyrolactone 75 75 75 7575 75 Sulfolane 3-methylsulfolane Ethylene glycol Silica Phosphoric acid1 Boric acid 1 p-Nitro benzoic acid 1 Polyethylene glycol 1 (Averagemolecular weight: 300) Electrolytic conductivity/ 23.85 23.81 23.8523.05 24.00 17.82 mS cm⁻¹ at 25° C. Withstand voltage/V at 165 165 165170 160 110 125° C.

In Table 3, from a comparison between Examples and Comparative Examplesin which the formulation of the electrolyte is substantially the same asthat of the corresponding Example except for the kind of the salt(Comparative Examples 5 to 10 correspond to Examples 3 to 8,respectively), it is found that every Examples have a higherelectrolytic conductivity and a higher withstand voltage value. As canbe seen from FIG. 3, Examples satisfy the following relationships offormulae (I):Y≧−7.5X+150, and X≧4, Y>0  (I).The electrolyte of the present invention is preferred in aluminumelectrolytic capacitors in any applications for low impedance grade orhigh rated voltage.(4) Evaluation of Device Properties and Appearance

Next, using the electrolytes in Examples 1, 3 and 4, aluminumelectrolytic capacitors each having a rated voltage for use of 100 V anda capacitance of 56 μF were prepared. Further, in Example 15, using anelectrolyte obtained by adding water in an amount of 3% by weight to aγ-butyrolactone solution of 25% by weight1-ethyl-2,3-dimethylimidazolinium tetrafluoroaluminate, an aluminumelectrolytic capacitor was prepared in the same manner as mentionedabove. The electrolyte used in Example 3 had a moisture content of 0.1%by weight as measured by means of a Karl Fischer moisture content meter.A capacitance at 120 Hz and an equivalent series resistance (ESR) at 100kHz were measured. In addition, the capacitors prepared were allowed tostand under no load at 125° C. for 500 hours, and then changes in thedevice properties and appearance were examined. On the other hand, whenusing the electrolytes in Comparative Examples 1, 5 and 6, preparationof a capacitor was impossible due to the low withstand voltage. Theresults are shown in Table 4.

TABLE 4 Example 1 Example 3 Example 4 Example 15 Capacitance/μF 54.854.8 54.8 54.5 Capacitance 55.9 53.7 55.2 38.4 (after unloading test)/μF Equivalent series 0.0066 0.0063 0.0105 0.0073 resistance/Ω Equivalentseries 0.0067 0.0063 0.0107 0.0545 resistance (after unloading test)/ΩAppearance Seal No change No change Seal (after unloading test) portionportion blistered blistered *Preparation impossible due to low withstandvoltage in Comparative Examples 1, 5 and 6.

As can be seen from Table 4, preparation of capacitors using theelectrolytes in Comparative Examples 1, 5 and 6 was impossible due tothe low withstand voltage, whereas, capacitors having excellent deviceproperties were able to be prepared using the electrolytes in Examples1, 3, 4 and 15. Especially in the capacitors using the electrolytes inExamples 1, 3 and 4, after the unloading test, almost no change wasfound in the device properties, indicating that the capacitors hadexcellent thermal stability. In addition, with respect to theappearance, in the capacitors using the electrolytes in Examples 1 and15, blistering was observed only in the sealing rubber portion,suggesting that gas generated in the capacitors, whereas, in thecapacitors using the electrolytes in Examples 3 and 4, such blisteringwas not found, indicating that the capacitors had more excellent thermalstability.

INDUSTRIAL APPLICABILITY

According to the present invention, an electrolyte for electrolyticcapacitor having high electrolytic conductivity and excellent thermalstability as well as excellent voltage proof property can be obtained.Further, by using this electrolyte for an electrolytic capacitor, anelectrochemical element having low impedance and excellent thermalstability as well as excellent voltage proof property can be obtained.

1. An electrolyte for an electrolytic capacitor containing a salt and asolvent, wherein the electrolyte satisfies the relationships of formulae(I):Y≧−7.5X+150, and X≧4, Y>0  (I) wherein X represents an electrolyticconductivity (mS·cm⁻¹) at 25° C., and Y represents a withstand voltage(V) of a capacitor.
 2. The electrolyte for an electrolytic capacitoraccording to claim 1, which further satisfies the relationships offormulae (II):Y≧−7.5X+150, and X≧8, Y>0  (II).
 3. The electrolyte for an electrolyticcapacitor according to claim 1, wherein the electrolyte contains 50% byweight or more of a solvent, and in the solvent, a weight ratio of asolvent having a boiling point of 250° C. or higher, a melting point of−60 to 40° C. and a permittivity (ε, 25° C.) of 25 or more is equal toor larger than a weight ratio of a solvent having a boiling point of190° C. or higher to lower than 250° C., a melting point of −60 to 40°C. and a permittivity (ε, 25° C.) of 25 or more.
 4. The electrolyte foran electrolytic capacitor according to claim 3, wherein the solventhaving a boiling point of 250° C. or higher, a melting point of −60 to40° C. and a permittivity (ε, 25° C.) of 25 or more in the solvent is asulfone.
 5. The electrolyte for an electrolytic capacitor according toclaim 4, wherein the sulfone is sulfolane or 3-methylsulfolane.
 6. Theelectrolyte for an electrolytic capacitor according to claim 1, whereinthe electrolyte contains 50% by weight or more of a solvent, and in thesolvent, a weight ratio of a solvent having a boiling point of 190° C.or higher to lower than 250° C., a melting point of −60 to 40° C. and apermittivity (ε, 25° C.) of 25 or more is larger than a weight ratio ofa solvent having a boiling point of 250° C. or higher, a melting pointof −60 to 40° C. and a permittivity (ε, 25° C.) of 25 or more.
 7. Theelectrolyte for an electrolytic capacitor according to claim 6 whichfurther satisfies the relationship of formula (III):Y≧−7.5X+220  (III).
 8. The electrolyte for an electrolytic capacitoraccording to claim 6, wherein the solvent having a boiling point of 190°C. or higher to lower than 250° C., a melting point of −60 to 40° C. anda permittivity (ε, 25° C.) of 25 or more in the solvent is at least onesolvent selected from the group consisting of carbonic esters,carboxylic esters, phosphoric esters, nitrites, amides and alcohols. 9.The electrolyte for an electrolytic capacitor according to claim 8,wherein the solvent having a boiling point of 190° C. or higher to lowerthan 250° C., a melting point of −60 to 40° C. and a permittivity (ε,25° C.) of 25 or more in the solvent is γ-butyrolactone or ethyleneglycol.
 10. The electrolyte for an electrolytic capacitor according toclaim 1, wherein the electrolyte contains a tetrafluoroaluminate ion.11. The electrolyte for an electrolytic capacitor according to claim 10,wherein the tetrafluoroaluminate ion is contained in the form of atleast one salt selected from the group consisting of quaternary oniumsalts, amine salts, ammonium salts and alkali metal salts oftetrafluoroaluminate.
 12. The electrolyte for an electrolytic capacitoraccording to claim 11, wherein the quaternary onium salt is at least onesalt selected from the group consisting of quaternary ammonium salts,quaternary phosphonium salts, quaternary imidazolium salts andquaternary amidinium salts.
 13. The electrolyte for an electrolyticcapacitor according to claim 11, wherein the quaternary onium salt has 4to 12 carbon atoms in total.
 14. The electrolyte for an electrolyticcapacitor according to claim 11, wherein a quaternary onium ion of thequaternary onium salt is at least one ion selected from the groupconsisting of tetraethylammonium, triethylmethylammonium,diethyldimethylammonium, ethyltrimethylammonium, tetramethylammonium,N,N-dimethylpyrrolidinium, N-ethyl-N-methylpyrrolidinium,1,3-dimethylimidazolium, 1,2,3-trimethylimidazolium,1-ethyl-3-methylimidazolium, 1-ethyl-2,3-dimethylimidazolium,1,2,3,4-tetramethylimidazolium, 1,3-diethylimidazolium,2-ethyl-1,3-dimethylimidazolium, 1,3-dimethyl-2-n-propylimidazolium,1,3-dimethyl-2-n-pentylimidazolium, 1,3-dimethyl-2-n-heptylimidazolium,1,3,4-trimethylimidazolium, 2-ethyl-1,3,4-trimethylimidazolium,1,3-dimethylbenzimidazolium, 1-phenyl-3-methylimidazolium,1-benzyl-3-methylimidazolium, 1-phenyl-2,3-dimethylimidazolium,1-benzyl-2,3-dimethylimidazolium, 2-phenyl-1,3-dimethylimidazolium,2-benzyl-1,3-dimethylimidazolium, 1,3-dimethylimidazolinium,1,2,3-trimethylimidazolinium, 1-ethyl-3-methylimidazolinium,1-ethyl-2,3-dimethylimidazolinium, 1,2,3,4-tetramethylimidazolinium,1,3-diethylimidazolinium, 2-ethyl-1,3-dimethylimidazolinium,1,3-dimethyl-2-n-propylimidazolinium,1,3-dimethyl-2-n-pentylimidazolinium,1,3-dimethyl-2-n-heptylimidazolinium, 1,3,4-trimethylimidazolinium,2-ethyl-1,3,4-trimethylimidazolinium, 1-phenyl-3-methylimidazolinium,1-benzyl-3-methylimidazolinium, 1-phenyl-2,3-dimethylimidazolinium,1-benzyl-2,3-dimethylimidazolinium, 2-phenyl-1,3-dimethylimidazoliniumand 2-benzyl-1,3-dimethylimidazolinium.
 15. The electrolyte for anelectrolytic capacitor according to claim 1, wherein the solvent is atleast one solvent selected from the group consisting of sulfolane andγ-butyrolactone, and 1-ethyl-2,3-dimethylimidazoliniumtetrafluoroaluminate or 1,2,3,4-tetramethylimidazoliniumtetrafluoroaluminate is added to the solvent in an amount of 5 to 40% byweight based on the weight of the electrolyte.
 16. The electrolyte forelectrolytic capacitor according to claim 1, wherein the electrolytefurther contains at least one additive selected from the groupconsisting of nitro compounds, phosphorus compounds, boron compounds,metal oxide particles, polyalkylene glycols and silicone oil.
 17. Theelectrolyte for an electrolytic capacitor according to claim 1, whereinthe electrolyte contains 1% by weight or less of moisture.
 18. Anelectrolytic capacitor using the electrolyte for an electrolyticcapacitor according to claim
 1. 19. An electrolytic capacitor which hasan anode electrode having an electrically insulating oxide film on theelectrode surface and a cathode electrode placed opposite thereto via aseparator, wherein the electrolyte held by the electrolyte for anelectrolytic capacitor according to claim
 1. 20. An electrolyticcapacitor electrolyte, comprising: a tetrafluoraluminate ion, whereinthe tetrafluoroaluminate ion is contained in the form of at least onesalt selected from the group consisting of quaternary onium salts, aminesalts, ammonium salts and alkali metal salts of tetrafluoroaluminate.21. The electrolytic capacitor electrolyte according to claim 20,wherein the quaternary onium salt is at least one salt selected from thegroup consisting of quaternary ammonium salts, quaternary phosphoniumsalts, quaternary imidazolium salts and quaternary amidinium salts. 22.The electrolytic capacitor electrolyte according to claim 20, whereinthe quaternary onium salt has 4 to 12 carbon atoms in total.
 23. Theelectrolytic capacitor electrolyte according to claim 20, wherein aquaternary onium ion of the quaternary onium salt is at least one ionselected from the group consisting of tetraethylammonium,triethylmethylammonium, diethyldimethylammonium, ethyltrimethylammonium,tetramethylammonium, N,N-dimethylpyrrolidinium,N-ethyl-N-methylpyrrolidinium, 1,3-dimethylimidazolium,1,2,3-trimethylimidazolium, 1-ethyl-3-methylimidazolium,1-ethyl-2,3-dimethylimidazolium, 1,2,3,4-tetramethylimidazolium,1,3-diethylimidazolium, 2-ethyl-1,3-dimethylimidazolium,1,3-dimethyl-2-n-propylimidazolium, 1,3-dimethyl-2-n-pentylimidazolium,1,3-dimethyl-2-n-heptylimidazolium, 1,3,4-trimethylimidazolium,2-ethyl-1,3,4-trimethylimidazolium, 1,3-dimethylbenzimidazolium,1-phenyl-3-methylimidazolium, 1-benzyl-3-methylimidazolium,1-phenyl-2,3-dimethylimidazolium, 1-benzyl-2,3-dimethylimidazolium,2-phenyl-1,3-dimethylimidazolium, 2-benzyl-1,3-dimethylimidazolium,1,3-dimethylimidazolinium, 1,2,3-trimethylimidazolinium,1-ethyl-3-methylimidazolinium, 1-ethyl-2,3-dimethylimidazolinium,1,2,3,4-tetramethylimidazolinium, 1,3-diethylimidazolinium,2-ethyl-1,3-dimethylimidazolinium, 1,3-dimethyl-2-n-propylimidazolinium,1,3-dimethyl-2-n-pentylimidazolinium,1,3-dimethyl-2-n-heptylimidazolinium, 1,3,4-trimethylimidazolinium,2-ethyl-1,3,4-trimethylimidazolinium, 1-phenyl-3-methylimidazolinium,1-benzyl-3-methylimidazolinium, 1-phenyl-2,3-dimethylimidazolinium,1-benzyl-2,3-dimethylimidazolinium, 2-phenyl-1,3-dimethylimidazoliniumand 2-benzyl-1,3-dimethylimidazolinium.
 24. An electrolytic capacitorelectrolyte, comprising: a tetrafluoroaluminate ion; wherein theelectrolyte contains 50% by weight or more of a solvent, and the solventis at least one solvent selected from the group consisting of carbonicesters, carboxylic esters, phosphoric esters, nitriles, amides,sulfones, alcohols and water.
 25. The electrolytic capacitor electrolyteaccording to claim 24, wherein the solvent contains at least one solventselected from the group consisting of sulfolane and 3-methylsulfolane inan amount of 40% by weight or more based on the total weight of thesolvents.
 26. The electrolytic capacitor electrolyte according to claim24, wherein the solvent contains at least one solvent selected from thegroup consisting of carbonic esters, carboxylic esters, phosphoricesters, nitriles, amides and alcohols in an amount of 40% by weight ormore based on the total weight of the solvents.
 27. The electrolyticcapacitor electrolyte according to claim 26, wherein the solventcontains at least one solvent selected from the group consisting ofγ-butyrolactone and ethylene glycol in an amount of 40% by weight ormore based on the total weight of the solvents.
 28. An electrolyticcapacitor electrolyte, comprising: a tetrafluoroaluminate ion and asolvent; wherein the solvent is at least one solvent selected from thegroup consisting of sulfolane and γ-butyrolactone, and1-ethyl-2,3-dimethylimidazolinium tetrafluoroaluminate or1,2,3,4-tetramethylimidazolinium tetrafluoroaluminate is added to thesolvent in an amount of 5 to 40% by weight based on the total weight ofthe electrolyte.
 29. An electrolytic capacitor electrolyte, comprising:a tetrafluoroaluminate ion; wherein the electrolyte further contains atleast one additive selected from the group consisting of nitrocompounds, phosphorus compounds, boron compounds, metal oxide particles,polyalkylene glycols and silicone oil.